US legal residents are not allowed to trade futures unless explicitly approved by the CFTC.
Is this a legal regulation, or an internal IB policy?

More...

What this means is that only approved foreign
futures can be traded. You can trade nikkei 225, australian,hong kong and some others. You can
trade those because they are approved. Some, like Korean kospi, aren't approved, so they can't be traded.

Ones that aren't approved are usually that way because the exchange that offers the contract has not gone through the process necessary to get the contract approved in the US.

You need to clean your glasses. This page is about foreign OPTIONS only:

"Securities OPTIONS are defined as any OPTION on an individual stock, or any cash settled broad based index future, US legal stock, or any cash settled broad based index future. US legal residents are generally excluded from trading securities OPTIONS outside of the United States due to SEC restrictions..."

You need to clean your glasses. This page is about foreign OPTIONS only:

"Securities OPTIONS are defined as any OPTION on an individual stock, or any cash settled broad based index future, US legal stock, or any cash settled broad based index future. US legal residents are generally excluded from trading securities OPTIONS outside of the United States due to SEC restrictions..."